As is well known, lasers are devices in which a material is stimulated in an optical cavity to produce amplified coherent light, normally in the form of a beam. The art is now sophisticated and various types of lasers are known, of which the following types are of interest in connection with the present invention, namely metal-vapor lasers, gas lasers and chemical-vapour lasers (including ionic and atomic lasers). In all such types of laser the source either is in the form of a gas or vapor or is brought to such a form before excitation. Metal-vapour lasers are of particular interest in that they afford a high theoretical efficiency, in terms of energy used for excitation and luminous output energy.
The energy levels involved in a laser are illustrated in FIG. 1. Particles at a ground energy level are exited to a resonant energy level and fall to a metastable level emitting coherent light. The efficiency of radiation is given by the formula ##EQU1## where r and m are indexes corresponding to the resonant and metastable levels,
E and g correspond to Energy and Statistical Weight levels, and
K is the transfer coefficient of energy from the excitation means.
The theoretical energy efficiency (which ignores the transfer coefficient K) can be calculated from the energy levels of the atomic configurations of the source material. Where the metastable level is high in relation to the ground and resonant energy levels, there is a relatively low theoretical efficiency. The theoretical efficiency increases as the metastable level approaches ground level, as shown in FIG. 1 in broken lines. The interest in metal-vapor lasers lies in the fact that in many metals there is a relatively low metastable energy level which can give rise to a high theoretical efficiency. This can lie between 20% and 80% depending on the particular metal concerned.
However, metal-vapour lasers are subject to the great difficulty of production of the vaporized metal. Normally this involves very high temperatures and this is especially the case with certain metals which have a high laser efficiency. Normally the metal is heated in an oven, which may involve inserting the whole or certain parts of the laser in the oven, with consequent disadvantages, or, more recently by repeated discharges. This is the method employed e.g. in Anderson et al [1] (see Schedule of References). Apart from practical difficulties, there are theoretical problems in that where high temperatures are used, population inversion becomes difficult if not impossible in the case of metals having low-energy, metastable levels on account of the high inherent polulation in the metastable energy level.
Another type of metal-vapor laser utilizes exploding wires (see for example Rice et al [2]. The disadvantage here is that the laser can only be used once. Yet another method involves a discharge between electrodes to strip a deposited metal off the tube wall (see Shukhtin et al [3]. This suffers from the drawback attendant also upon the other methods that it is necessary to utilize conductor wires passing through the tube, which is expensive and may lead to the introduction of impurities. Furthermore, the metal-vapor fills the whole tube which may not always be desirable. It can lead for example to deposition of the metal on the beam-exit glass.
It is also generally very difficult to create a high atomic (or ionic) vapor density by reason of the high vaporization temperatures of most metals. Furthermore, quick vaporization is normally possible to achieve.
In particular it has not hitherto been possible to obtain a laser utilizing the 4511.32 A transition of indium, whose theoretical efficiency is over 80%.